1. Field of the Invention
The present invention relates to a thermostat device that variably controls the water temperature in a cooling water temperature control system that variably controls the cooling water temperature of, for example, an internal combustion engine (“engine” hereinafter) used in an automobile and the like. More particularly, the present invention relates to an assembly structure of a device housing of a thermo element assembly of the thermostat device.
2. Description of the Related Art
A thermostat device that is disposed in, for instance, a cooling water system of an engine incorporates a wax (thermal expansion unit) that expands/contracts upon detecting a change in temperature of the cooling water within a circulation flow path. The thermostat device functions to open/close a valve unit in response to a change in the volume of the expanding/contracting wax, to keep the cooling water at a predetermined temperature.
As this type of thermostat device, there is known the one with a structure that has, for example, a piston that is installed and fixed within a housing coupled to a plurality of flow paths, a cylinder container that reciprocates with respect to the piston, a thermal expansion unit that is provided within the cylinder container and changes its volume with a change in temperature, to reciprocate the cylinder container, and a heating element that is provided within a casing of the piston and heats the thermal expansion unit when supplied with current, wherein the heating element is configured by an extension member that has thermal conductivity and is formed to pass through the inside of the casing of the piston from the outside of the same, a heating part that is formed inside the extension member on the inside of the casing, and an electrode part that is formed on the extension member on the outside of the casing and is electrically connected to the heating element, and wherein a terminal that supplies voltage to the heating part abuts on the electrode part, so that a voltage supply source and the heating part are electrically connected with each other (see Japanese Patent Application Laid-open No. 2005-155831, for example).
Such a configuration has the following advantages. Since the terminal supplied with voltage is in abutment with the electrode part of the extension member and configured to be detachable with respect to the heating element, the heating element can be assembled and maintained readily. Furthermore, because the heating part of the heating element is formed within the extension member and only the electrode part applied with voltage is formed on the extension member, a highly durable heating element that does not break easily can be obtained. In addition, the heating element provided within the casing of the piston can generate heat substantially evenly from the circumferential surface of the piston, whereby the thermal expansion unit can be heated effectively.
However, in the conventional thermostat device described above, it is troublesome and complicated to assembly the piston, the heating element, the cylinder container, the terminal and the other components to the housing, and such a troublesome work leads to a cost increase.
Moreover, in this conventional device, the piston is inserted into a sealing member and then directly into the housing and held by the metallic materials and elastic rubber. Therefore, such metals as the thermo element and the housing cannot be fixed by press-fitting. Consequently, the flow of the cooling water could tilt the thermo element section especially when the valve is opened. In addition, this conventional device has a reliability problem that a load resulting from the activation of the thermo element or after-mentioned water pressure is applied to the electrically connected section between the terminal of the heater inside the piston and the thermo element section, deteriorating the electrical connection.